Horizontal line hydrophone arrays are widely used in underwater seismic exploration and military sonar systems, being towed behind a ship. Usually, they contain many omnidirectional sensors whose signals are processed to form beams that can be electronically steered to any direction. However, because the array is omnidirectional about its axis, there is ambiguity as to whether a received signal is coming from the left-hand or right-hand side of the line. The line array, being towed horizontally, may also be subject to random twists throughout its length, and so the vertical orientation of the individual elements of the array cannot be controlled. This does not matter if the sensors are omnidirectional, and the resulting left-right ambiguity is acceptable; however, for most systems, performance would be much improved by resolving the ambiguity.
This left-right ambiguity has previously been resolved in one of two ways: either by maneuvering the ship so as to take a second reading from a different position, or by installing directional sensors in place of the omnidirectional sensors in the line.
The firsr method is, however, very time consuming and unwieldy. The second requires expensive hardware; because the line hydrophone array is long and flexible, the orientation of individual sensors about the line axis cannot easily be controlled. Each of the individual sensors must comprise an orthogonal pair of dipole hydrophones, for sensing acoustic acceleration, velocity, or pressure gradient; an omnidirectional monopole hydrophone, for sensing pressure; and a tilt sensor, the signals from these being electronically processed to form a single directional element in the line array. If the processing is done in the array, the electronics in the array are complex and expensive. If the processing is done on the ship, four separate signals must be carried by the tow-cable to the ship for each sensor group.
One of the simplest directional hydrophones, used in the above sensors, consists of a dipole hydrophone in combination with a monopole hydrophone. The dipole hydrophone senses a horizontal vector component of th4e acoustic field (velocity, acceleration or pressure gradient), and the monopole hydrophone senses a scalar component (pressure). The two signals are added, with appropriate phase and amplitude adjustment, to form right-facing and left-facing cardioid directivity patterns: EQU P(.theta.,.phi.)=P[1+sin (.theta.) sin (.phi.)] EQU P(.theta.,.phi.)=P[1-sin (.theta.) sin (.phi.)]
where .theta. is the angle from the vertical, .phi. is the azimuth angle, and P is a reference amplitude.
The existence of a suitable crossed dipole sensor is assumed in the following disclosure. Each dipole must have a differential output (as opposed to a single-ended output) and its electrical impedance must be essentially capacitive, but it does not matter which vector component of the sound field is detected, as this merely affects the phase and amplitude adjustment of the signals before they are added together.